Two devices from Yemen bound for the US have been intercepted by security officials in the UK and Dubai. An international investigation is taking place in the US, Europe and the Middle East.

How were the bombs transported?

The two devices were posted in the Yemeni capital Sanaa with freight firms UPS and FedEx. Initially it was thought that both packages were flown out on cargo planes. But Qatar Airways later confirmed that the bomb intercepted in Dubai had been transported on two of its passenger jets, first from Yemen to Doha and then on to Dubai on a second plane. The first leg would have been on an Airbus A320, and the second leg on an A320, A321 or Boeing 777.

The device seized in the UK also went via Dubai and is believed to have passed through Cologne in Germany, before being intercepted at East Midlands Airport in the UK. It was transported by UPS. The exact route and details of which planes were involved has not been confirmed.

Both parcels were addressed to synagogues in Chicago, although the UK later said it was believed that the device seized in England had been designed to blow up on the plane itself.

How were the bombs discovered?

The crucial tip-off that led to their discovery came from a repentant al-Qaeda member, Jabr al-Faifi, who handed himself in to the Saudi authorities two weeks ago, UK officials say.

He is said to be a former detainee at the US detention centre in Guantanamo Bay, Cuba.

After leaving Guantanamo, he went through a rehabilitation programme in Saudi Arabia and then rejoined al-Qaeda in Yemen before turning himself in to Saudi authorities.

Police in Dubai have confirmed that the device seized there contained the powerful PETN explosive.

The device intercepted in the UK was viable and could have blown up an aircraft, said UK Home Secretary Theresa May.

Both Mrs May and the Dubai authorities said the devices bore all the hallmarks of al-Qaeda - an apparent reference to the group's previous use of PETN in other attempted attacks.

The Dubai package also contained a closed electrical circuit connected to a mobile phone SIM card hidden inside the printer. The printer was in a box with a textbook on management, a copy of The Mill on the Floss by George Eliot and various handicrafts, including a pink and purple lidded basket.

Earlier, US Congresswoman Jane Harman told the New York Times that both packages contained computer cartridges filled with PETN.

Ms Harman, who was briefed by the US Transportation Security Administration on Friday, said one device used a mobile phone as a detonator, while the other had a timer.

What is PETN?

Pentaerythritol tetranitrate (PETN) is a powerful explosive, which is part of the same chemical family as nitroglycerin. It is manufactured as a white powder and is often a component of plastic explosives like Semtex. It is relatively stable and used primarily as a high explosive. It is also used in medicine as a vasodilator to treat angina pectoris.

It has many manufacturers, including some in the US.

PETN has been used in a number of attacks linked to al-Qaeda, and its Middle Eastern offshoot linked to al-Qaeda in the Arabian Peninsula (AQAP). As well as the suspect packages intercepted in the UK and Dubai, PETN was also found on the person of Umar Farouk Abdulmutallab. He was arrested in December 2009, suspected of attempting to blow up an airliner flying from Amsterdam to Detroit.

Screening of air freight differs from country to country. Some countries are rigorous, others screen a small sample of cargo, some have no screening at all, and some screen only to make sure that the consignment sheet matches the items: for example, if the consignment sheet says TVs, the screening checks that TVs are in the boxes.

It is unclear if any screening takes place in Yemen.

What happens in the US?

In August 2010, the US introduced a requirement for 100% of cargo loaded onto passenger planes to be screened for explosives. Certified equipment and companies are used. The US uses advanced X-ray machines which give more than one view of the package. Large consignments must be broken down into smaller boxes, although the x-ray machines can be quite large, allowing packages the size of a refrigerator to be screened.

Explosives trace detection is also used, in the same way as passengers' items are sampled with a swab on a random basis. Freight loaded on non-passenger flights is also screened.

What about screening in the UK?

The UK uses a system of "known consignor". This means a company is audited by the Department for Transport and given status as being a trusted shipper and their cargo is trusted to be safe and not subject to screening. The president of detection equipment company Smiths Detection, Stephen Phipson, said "quite a lot of cargo is not searched" as a result of this system.

For cargo which is shipped by non-trusted shippers, other companies screen it using sniffer dogs and basic X-rays.

Mr Phipson said the UK system was known as being "fairly rigorous", but could be upgraded to the more advanced US system.

Why is there so much concern about Yemen?

Security services in the UK and US fear Yemen is becoming a haven for militants, following the crackdown on al-Qaeda affiliates in Afghanistan and Pakistan.

It is the base for al-Qaeda in the Arabian Peninsula (AQAP) and also harbours Anwar al-Awlaki, a radical Islamist cleric born in the US. He has links to the suspect in the Detroit bomb plot, Umar Farouk Abdulmutallab.

Yemeni forces have just completed an offensive in the remote Shabwa province, says BBC Middle East correspondent Jon Leyne in Cairo. They failed to find the members of al-Qaeda in the Arabian Peninsula they were targeting, but a senior officer said there was evidence the group had been in the area a few days earlier.

Al-Qaeda in the Arabian Peninsula (AQAP) was formed in January 2009 by a merger between two regional offshoots of the international Islamist militant network in Yemen and Saudi Arabia.

It has fast gained a reputation for daring and unusual attacks, being called "the most active operational franchise" of al-Qaeda outside Afghanistan and Pakistan by the White House's counter-terrorism advisor John Brennan.

AQAP was founded by two lower-ranking al-Qaeda members who escaped from a Yemeni jail in 2006. One of them, Nasser Abdul Karim al-Wuhayshi, is a former personal assistant to al-Qaeda leader Osama Bin Laden in Afghanistan.

In September 2008, AQAP attacked the US embassy in the Yemeni capital, Sanaa, using car bombs and mortars.